This invention relates to pneumatic radial tires containing steel cords as a reinforcement, and more particularly to pneumatic radial tires having improved fatigue resistance and rolling resistance by reinforcing with steel cords each having a flat or elliptic section.
As reinforcements for this type of tire, there have hitherto been widely used steel cords of a strand construction wherein two or more circular filaments are twisted to form a strand. Then, a plurality of the resulting strands are twisted with each other, steel cords of a bundle-twisted construction wherein plural filaments are arranged concentrically and then twisted with each other, and the like. However, the strand construction cords have drawbacks in that the utilization of filament strength is low and fretting is apt to occur though the input in the deformation is small, while the bundle-twisted cords have drawbacks in that the input in the deformation is large and hence material fatigue is apt to occur through the utilization of filament strength is high and fretting hardly occurs. Among these cords, strand construction cords are particularly produced through a complicated twisting step, so that productivity is poor. In order to improve such productivity, therefore, there is a need to simplify the cabling step in a simple structure.
Now, steel cords for tire reinforcement are required to have high strength, excellent fatigue resistance and the like. In the latest radial tires having a low aspect ratio or the like, it becomes particularly important to mitigate the input of strain in the side portion of the tire. That is, the improvement of fatigue resistance in the steel cord is increasingly required together with high strength. In addition, there is a requirement to render the tire building workability (the bending property of steel cord) satisfactory. For this purpose, it is desired to twist a large number of filaments having a smaller diameter with each other, but such a twisting construction deteriorates the productivity of steel cords from the viewpoint of economy and also, the strength of the steel cord reduces.
In order to solve the above drawbacks, steel cords, wherein two or more filaments having a circular section or an irregular section other than circle are arranged in parallel with each other and wrapped with a spiral filament, have been proposed in Japanese Utility Model laid open No. 70,002/78. However, such steel cords have the following drawbacks. That is, when the cord is subjected to a bending deformation, the inside portion of the cord against the bending is compressed, while the outside portion against the bending is stretched. Hence, the cord is apt to be subjected to material fatigue due to the fact that filaments in the cord are arranged in parallel with each other, and in some cases the filaments in the inside portion against the bending are apt to be folded and finally the cord is broken.
Now, the inventors have previously proposed a steel cord wherein two or three strands each composed of plural filaments are arranged in parallel with each other to form a core and plural filaments are twisted around the core and wrapped with a spiral filament as disclosed in Japanese patent application No. 119,208/79 laid open Apr. 21, 1981. However, some of the resulting steel cords are very poor in the twisting stability, so that workability is deteriorated during the rubber coating for steel cord, or rotational disorder is produced in steel cords 2 embedded in rubber 1 as shown in FIG. 1a to easily cause the distortion of the rubberized cord layer. That is, they have frequently a bad influence on the tire building workability.